The molecular mechanisms underlying the steps of the human T-cell leukemia virus type 1 (HTLV-1) infectious cycle are poorly understood. Experimental and epidemiological evidence supports a model in which HTLV-1 is transmitted directly between T lymphocytes. We predict that virus assembly, release, transmission, and replication are coordinated with cell-cell adhesion events. Thus, we are examining the components and pathways that control dissemination and replication of HTLV-1 in cell culture model systems. Biochemical, functional, and electron microscopic methods revealed that a peptide motif in the matrix domain of HTLV-1 Gag is bound by a cellular NEDD4-family ubiquitin ligase (WWP1) that is involved in membrane protein trafficking. Ubiquitination of the viral Gag protein by WWP1 is an essential early step in virion assembly and release. We also showed that HTLV-1 Gag protein is targeted to tetraspanin enriched microdomains (TEMs) in the plasma membrane. In fact, HTLV-1 Gag interacts with the cytoplasmic loop of the tetraspanins CD81 and CD82. This finding is important, because tetraspanins help organize cell surface adhesion proteins and play important roles in cell adhesion and signal transduction. The association of HTLV-1 virion components with tetraspanins and other cell surface and cytoskeletal proteins may be necessary to direct virion transmission to sites of cell-cell adhesion. After entry into the target cell, the viral enzymes and structural proteins form a replication complex that converts the single-stranded viral genomic RNA into a double-stranded DNA, which is then integrated into the host cell chromosome. We have developed cell culture systems, infectious molecular clones, and viral vectors in order to study mechanisms of HTLV-1 infection and to determine whether cellular factors can enhance or restrict replication. We have also characterized the virion-associated reverse transcriptase and have begun to define the properties of the recombinant protein. We are studying the mechanisms that HTLV-1 has evolved to evade cellular defense mechanisms that restrict the replication of other retroviruses. We have shown that HTLV-1 counteracts the antiviral effects of APOBEC3G by a cis-acting virion exclusion mechanism, which is very different compared with the way HIV-1 evades APOBEC3G. In contrast to HIV-1, HTLV-1 was not sensitive to restriction by cellular TRIM5alpha or other TRIM proteins. Although the basic schemes for transmission and replication are conserved among retroviruses, variations on common themes are likely to reveal important insights into the strategies that different viruses use for propagation and persistence in vivo. In this respect, it is useful to compare HTLV-1 and HIV-1, because both are T-cell-tropic retroviruses whose very different biological properties may be traced back to divergent strategies for escaping host-mediated immune and cellular defense mechanisms.